In recent years, much effort has been expended in the metallurgical field to develop techniques for continuously casting molten metal into continuous metal rod. The advantages of continuous casting over batch casting are well known in the art and include the elimination of such prior art batch casting operations as initially casting individual bars of the metal, cooling the bars and casting molds, stripping the bars from the casting molds, and then reheating the bars to a sufficient temperature so that they may be rolled into lengths of rod.
For example, the prior art production of copper rod from cast copper wire bars, typically weighing 250 pounds, included reheating the cast bars to 1700.degree. F in order to homogenize the metal and condition it for the subsequent hot-forming operation, and thereafter rolling the bars in a so-called "looping mill" which was a long, slow operation that permitted the rod to completely recrystallize between rolling passes. After rolling, the copper rod would be completely covered with a black oxide coating, and no cold work (stored energy) would remain in the rod as it exited the mill at 1000.degree. F. The 250 pound length of rod emitting from the looping mill was taken-up on a coiling device and immediately quenched to facilitate subsequent handling. Inasmuch as no stored energy remained in the rod as it exited the mill, the quenching operation did not affect its metallurgical properties.
In the continuous production of metal rod according to present practice, on the other hand, molten metal passes from a holding furnace into the mold of a casting wheel where it is cast into a metal bar. The solidified metal bar is removed from the casting wheel and directed into a rolling mill where it is rolled into continuous rod. Depending upon the intended end use of the metal rod product, the rod may be subjected to cooling during rolling or the rod may be cooled or quenched immediately upon exiting from the rolling mill to impart thereto the desired mechanical properties. As disclosed in U.S. Pat. No. 3,395,560 to Cofer et al., a continuously-processed rod is preferably cooled as it exits the rolling mill and prior to being coiled. Because the continuous casting and rolling operation does not include the intervening homogenizing step of the prior art batch casting of wire bars, and because the rolling operation is relatively rapid as compared with the prior art looping mill, the continuous rod emitting from the rolling mill will have a substantial amount of cold work retained therein and thus the immediate quenching operation will serve to retain the same and freeze impurities in solid solution thus improving the tensile strength of the product.
Conventional cooling techniques include immersing the rod in a coolant, and spray-cooling the rod with coolant. In all cases, however, it is standard practice to cool or quench the continuously produced rod prior to its coiling and rods formed in this manner are hereinafter referred to as cold-coiled rods in contrast to the hot-coiled rods of the present invention.
In some instances, it is desirable to have uniformly dispersed copper oxide inclusions throughout the rod product whereas in other instances, it is necessary to remove such oxide by shaving or scalping operations. The oxide and other surface scale may be removed from the rod product by pickling the surface of the rod in a liquid such as sulfuric, nitric or other acids. The pickling operation also performs a cooling function so that it is possible to both cool and pickle the rod in one operation and one example of such a system for quench-pickling cast rod is shown in U.S. Pat. No. 3,623,532 to Cofer et al. Rods formed in this manner are also cooled, due to the pickling operation, prior to their delivery to a coiling apparatus and therefore are cold-coiled.
One disadvantage of the prior art systems for continuous production of metal rod is that due to the cooling operation, the rod becomes harder and hence more difficult to coil. This is particularly disadvantageous with large diameter rod. Another disadvantage of quenching the high temperature rod prior to coiling is that the retained vacancies and lattice defects which are present after quenching remain in the rod since the temperature of the quenched rod is too low to enable these defects to be rectified through thermal vacancy diffusion. For many applications, such as wire drawing, rod which is quenched prior to coiling becomes too hard and will have too high a yield tensile strength and too high a degree of residual stress to be commercially suitable.
Additionally, rod quenched at high temperatures as it exits the rolling mill will exhibit a high recrystallization temperature because impurities contained in the metal will be trapped or frozen in solid solution. Consequently, the rod will have a high annealing temperature which obviously necessitates appropriate process equipment and energy requirements capable of effecting the high temperature anneal. Moreover, when the rod is drawn into wire intended to be subsequently fabricated into magnet wire by coating the same with an enameling composition in an annealing tower, a high temperature anneal (e.g., greater than 500.degree. F) will cause the enamel to blister on the surface of the wire. Consequently, under these circumstances the annealing and enameling would have to be effected in separate operations.